Sen. Joe Donnelly of Indiana today becomes the first Democrat who voted for the health-care law to back changes to its requirement that companies offer coverage to employees working 30 hours a week or more or pay a penalty.

Mr. Donnelly’s staff said the senator plans to announce Wednesday afternoon that he is signing on as a co-author to a bill initially introduced by Republican Sen. Susan Collins of Maine in April that seeks to change the rule to apply only to workers clocking 40 hours a week or more. The rule kicks in next year.

As we’ve reported, employer groups trying to get the rule peeled back had been searching for a long time for any Democrats to openly support them, and their prospects appeared to be dimming.

Most Democrats are still maintaining a unified front in support of the law’s provision — which can’t easily be altered without raising the price-tag of the overhaul. Big backers of the requirement, like Sen. Tom Harkin of Iowa, argue that employers that try to avoid giving health coverage to their workers are making short-sighted business decisions.

Supporters also point enthusiastically to stories like this one that show some companies have chosen to offer coverage to improve recruitment and retention of employees.

But some employer groups are celebrating today. “We are pleased lawmakers are finally heeding our call,” said Steve Caldeira, president of the International Franchise Association, one of the trade groups that had been pushing hard to find a Democrat to support its move to raise the threshold at which employees have to be given insurance.

The Indiana senator is expected to point to employers curbing workers’ schedules to minimize their exposure to the requirement when he explains his decision to join Ms. Collins in reintroducing her bill. Mr. Donnelly, who was elected to the Senate in November, voted for the health-care law when he was a member of the House.

“Many employers have to make decisions because of this definition, and some have chosen to cut current part-time employees’ hours. This leaves too many Indiana families struggling to make ends meet,” he said in a statement Tuesday night.

Ms. Collins and Mr. Donnelly are also writing to President Barack Obama asking him to delay enforcing the rules in the first year, which they argue he can do through executive authority.

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